Over the past 50 years, the field of organic photochemistry has produced a wealth of information, from reaction mechanisms to useful methodology for synthetic transformations. Many technological innovations have been realized during this time due to the exploits of this knowledge, including photoresists and lithography for the production of integrated circuits, photocharge generation for xerography, multidimensional fluorescence imaging, photodynamic therapy for cancer treatment, photoinitiated polymerization, and UV protection of plastics and humans through the development of UV absorbing compounds and sunscreens, to name a few.
The scientific basis of many of these processes continues to be utilized today, particularly in the development of organic three-dimensional optical data storage media and processes.
With the ever-pressing demand for higher storage densities, researchers are pursing a number of strategies to develop three-dimensional capabilities for optical data storage in organic-based systems. Among the various strategies reported are holographic data storage using photopolymerizable media (Cheben, P. and Calvo, M. Appl. Phys. Lett. 2001, 78, 1490-1492; U.S. Pat. No. 5,289,407 and U.S. Pat. No. 6,310,850), photorefractive polymers (Belfield et al. Field Responsive Polymers, ACS Symposium Series 726, ACS, 1999, Chapter 17 pages 250-263), and two-photon induced photochromism (Belfield et al, “Organic Photorefractives, Photoreceptors, and Nanocomposites,” Proc. SPIE Vol. 4104, 2000, 15-22; U.S. Pat. No. 5,268,862). It is known that fluorescent properties of certain fluorophores may be changed (quenched) upon protonation by photogeneration of acid (Kim et al, Angew. Chem. Int. Ed. 2000, 39, 1780-1782). Belfield et al. J. Phys. Org. Chem. 2000, 13, 837 has reported two-photon induced photoacid generation using onium salts and short pulsed near-IR lasers in the presence of a polymerizable medium, resulting in two-photon photoinitiated cationic polymerization. The inherent three-dimensional features associated with two-photon absorption provides an intriguing basis upon which to combine spatially-resolved, two-photon induced photoacid generation and fluorescence quenching with two-photon fluorescence imaging.
Three-dimensional (3-D) optical data storage based on two-photon processes provides a mechanism for writing and reading data with less crosstalk between multiple memory layers, due to the quadratic dependence of two-photon absorption (2PA) on the incident light intensity, as reported by D. A. Parthenopoulos, et al. in Science, Vol. 245, page 843 (1989) and J. H. Strickler, et al in Opt. Letters, Vol. 16, p. 1780 (1991). This capacity for highly confined excitation and intrinsic 3-D resolution afford immense information storage capacity (up to 1012 bits/cm3) according to D. A. Parthenopoulos, et al. in Science, supra. Recently, the use of photochromic materials for 3-D memory has received intensive interest because of several major advantages over current optical systems, including an erasable/rewritable capability, high resolution, and high sensitivity, as reported by S. Kawata et al., in Chem. Reviews, Vol. 100, page 1777 (2000).
Among the several classes of photochromic materials, diarylethenes with hetrocyclic aryl groups are the most promising candidates for applications because of their excellent fatigue resistance, picosecond switching time, high photoisomerization quantum yields, and absence of thermal isomerization, as discussed by S. Tian et al. in Chem. Soc. Rev., Vol. 33, page 85 (2004) and M. Irie, in Chem. Reviews, Vol. 100, page 1685 (2000).
Various optical systems for reading and writing 3-D memories using diarylethene derivatives as storage media have been reported by S. Kawata et al., in Chem. Reviews, supra wherein several methods using fluorescence readout were used to avoid destructive readout as reported by M. Irie in Chem. Reviews, supra.
In particular, Jares-Erijman in J. Am. Chem. Soc. Vol. 124, 7481-7489 (2002) and Irie in Chem. Reviews, supra reported using Lucifer Yellow 1 as the donor and bis(thienyl)ethane as the acceptor to build fluorescent molecules and developed a general conceptual reading/writing system based on fluorescence resonance energy transfer (FRET), where they found that the single-photon fluorescence emission of the donor is reversibly modulated by cyclical transformations of the photochromic acceptor upon irradiation of appropriate UV and visible light, respectively.
The Irie et al. system provided a novel method of using fluorescence to readout the recorded data without simultaneously erasing part of the stored information. However, the modulation of the two-photon fluorescence emission of a dye by a photochromic diarylethene has not been reported as the read-out method in a 3-D optical data storage system. This may be due, in part, to the difficulty in making suitable materials with large two-photon absorption (2PA) cross-sections, high fluorescence quantum yields, and high photostability, in which the emission spectrum properly overlaps the absorption spectrum of one of the isomers of the photochromic diarylethene.
Thus, in prior art demonstrations of two-photon 3-D optical data storage in photochromic materials, the optical storage system has traditionally failed because read out of the stored data caused some data erasing, since the photochrome itself was irradiated both in the writing step and read out step. Even in single-photon data recording with single-photon-induced fluorescence resonance energy transfer to a photochromic material, the single-photon recording and readout only allows one layer of data. This is a serious limitation on the amount of data that can be stored.
For practical applications, non-destructive read-out capability is indispensable. Appropriate read out methods such as refractive index changes, fluorescence, phosphorescence, and the like, should be selected in order to read the stored data without simultaneously erasing part of the stored information.
So, in addition to high data storage volume and fast readout, there is a need for data storage materials that are stable, highly responsive, exhibit high sensitivity and fidelity, and are less complex. In addition, the data storage and readout processes must also be more straight forward (less complex) and reliable. As mentioned above, the previously developed systems fall short in these regards.